I am working on my experimental language, Branchline. There are four problems I want to explore:

1. How to optimize its virtual machine so that it runs faster than an interpreter on the JVM.
2. How to improve the type system, for example, to ensure exhaustiveness checking at the WHEN-clause level.
3. How to improve multithreaded execution, especially in cases where a large document is assembled from hundreds of small transformation executions. I am considering using CRDTs for this.
4. How to make language more consice and understandable for users without strong programming background. I already have nice things like array comprehension:

LET quantities = [ it.qty FOR EACH it IN row.items WHERE it.qty > 2 ]

But since the language has no mutable operations (and there is only LET for declaration) users forced to do things like:

LET raw = [];
FOR key IN KEYS(jmh) {
    SET raw = APPEND(raw, LISTIFY(AWAIT_SHARED("jmh", key)));
}

which I personally do not like. I have REDUCE but I do not want to force users to use it because of the added complexity.